The 'Windows' key (because I don't have it - but if I had it wouldn't use it!).

On a second thought if I had it, then I would either switch back to a keyboard that doesn't have it. Or extirpate it!!

Oh, and before someone points out that "since I don't have it it can't be the most useless one", I'll say that quite on the contrary, any element of the empty set, just because of its emptiness does have any property you want. In particular, being the set of 'Windows' keys on my keyboard empty, all of its elements are the most useless keys. And the most useful, d'oh! For some reason I tend to concentrate on the former...